Is it formatted HFS+ Journaled? Is File Sharing enabled in AirPort Utility > Disks? Can you mount it in Finder? Is your AEBS running the latest firmware?

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All of the above are required for this combo to work. I have had it working for quie some time, and it works flawlessly. To get the disk to automount, just drag it (the TM disk, not the AEBS) from the finder to the startup items in System Preferences.

NOTE: I just reread your title. Did you mean AEBS (Airport Extreme Base Station) or AE (Airport Express)? The AE does not support this functionality, it requires an AEBS.

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